1. Field of the Invention
The present invention relates to an aluminum nitride sintered body, and more particularly, it relates to an aluminum nitride sintered body allowing low-temperature sintering and having high strength and high thermal conductivity. The invention further relates to a method of preparing the same.
2. Description of the Prior Art
Aluminum nitride (AlN) having high thermal conductivity and a low thermal expansion coefficient is recently employed as the material for insulated substrates for various types of electronic components, in place of alumina which has been employed in general.
In general, however, aluminum nitride has a relatively high sintering temperature of at least 1800xc2x0 C. An existing sintering furnace or jig component cannot sufficiently withstand such a high temperature but must be frequently repaired or discarded/exchanged. Further, aluminum nitride sintered at a high temperature requires high sintering energy. Therefore, the cost for an aluminum nitride sintered body is higher than that for an alumina sintered body; which hinders to hinder popularization of aluminum nitride.
In order to sinter aluminum nitride, which is generally hard to sinter as compared with alumina, a sintering aid of an alkaline earth metal element compound or a rare earth element compound is mainly employed. Particularly in order to lower the sintering temperature, more specifically to enable sintering at a temperature of not more than 1700xc2x0 C., combined use of an alkaline earth metal element compound and a rare earth element compound is studied. Typically, various studies have been made on sintering aids prepared by combining calcium compounds and yttrium compounds.
For example, Japanese Patent Laying-Open No. 61-117160 (1986) describes an aluminum nitride sintered body obtained by normal pressure sintering under a temperature of not more than 1700xc2x0 C. with a sintering aid prepared by combining an alkaline earth metal element compound such as CaCO3 and a rare earth element compound such as La2O3. Japanese Patent Laying-Open No. 63-190761 (1988) describes a sintering aid for aluminum nitride prepared by combining CaO and Y2O3.
A technique of reducing an aluminum oxide contained in a sintered body with carbon or a material liberating carbon for improving the thermal conductivity of an aluminum nitride sintered body is generally known. For example, each of Japanese Patent Publication Nos. 7-5372 to 7-5376 (1995) discloses a method of increasing the thermal conductivity of aluminum nitride by nitriding an oxide contained therein through free carbon with a sintering aid of an yttrium compound. Further, Japanese Patent Laying-Open No. 58-55377 (1983) describes a method of reducing/removing oxygen by employing an alkaline metal compound as a sintering aid and adding carbon powder or the like.
In addition, it is known that a thick metallized film having high strength can be formed by introducing a rare earth element or an alkaline earth metal element into an aluminum nitride sintered body. For example, Japanese Patent Publication No. 5-76795 (1993) discloses a circuit board obtained by forming a conductor part or a dielectric part prepared from at least either paste containing Ag or paste containing Au on an aluminum nitride sintered body containing at least one element selected from a rare earth element and an alkaline earth metal element. Japanese Patent Publication No. 7-38491 (1995) describes a method of forming a conductive layer of a high melting point metal such as tungsten or molybdenum on an aluminum nitride sintered body containing at least one element selected from a rare earth element and an alkaline earth metal element.
As described above, sintering of aluminum nitride under a low temperature of not more than 1700xc2x0 C. has been enabled due to development of a new sintering aid prepared by combining an alkaline earth metal element compound and a rare earth element compound. Thus, the thermal conductivity of an aluminum nitride sintered body is improved, and such an aluminum nitride sintered body is increasingly applied to a substrate for an exothermic semiconductor element such as a power device.
In the aforementioned method employing the sintering aid of a rare earth element and/or an alkaline earth metal element, however, a rare earth aluminum oxide, an alkaline earth aluminum oxide, a rare earth alkaline earth aluminum oxide and the like are formed between an oxide present in the aluminum nitride sintered body and the sintering aid. Although formation of these oxides is necessary for the aforementioned low-temperature sintering under a temperature of not more than 1700xc2x0 C., the grain sizes of the sintered body are increased due to the oxides.
In recent years, aluminum nitride is frequently applied to a radiating substrate for a power module or a jig for semiconductor equipment, which is used under a strict heat cycle. Therefore, aluminum nitride must be improved in thermal shock resistance as well as strength for serving as ceramic. In this regard, the mean grain size of the aluminum nitride sintered body must be not more than 3 xcexcm, preferably not more than 2 xcexcm. In the conventional method, however, further improvement of the strength of the sintered body cannot be attained due to increase of the grain sizes resulting from formation of a large amount of oxides.
In consideration of such general circumstances, an object of the present invention is to provide an aluminum nitride sintered body excellent in thermal shock resistance and strength and applicable to a radiating substrate for a power module or a jig for semiconductor equipment used under a strict heat cycle by suppressing grain growth in the case of employing a rare earth element and an alkaline earth metal element as materials for a sintering aid and a method of preparing the same.
In order to attain the aforementioned object, the inventors have deeply studied this matter to find that grain growth can be suppressed and thermal shock resistance and strength of an aluminum nitride sintered body can be remarkably improved even if employing a sintering aid containing a rare earth element and an alkaline earth metal element by properly selecting the amounts of blending thereof and controlling the amount of carbon remaining in the sintered body, to propose the present invention.
The aluminum nitride sintered body according to the present invention contains at least 0.005 percent by weight and not more than 0.1 percent by weight of carbon, at least 0.01 percent by weight and not more than 5 percent by weight of an alkaline earth metal element in terms of an oxide thereof and at least 0.01 percent by weight and not more than 10 percent by weight of a rare earth element in terms of an oxide thereof with a rest mainly composed of aluminum nitride.
Preferably, the alkaline earth metal element includes at least one element selected from a group consisting of Ca, Sr and Ba.
Preferably, the rare earth element includes at least one element selected from a group consisting of Y, La, Ce, Sc, Yb, Nd, Er and Sm.
Preferably, the mean grain size of aluminum nitride grains forming the sintered body is not more than 3 xcexcm.
Preferably, the aluminum nitride sintered body further comprises a conductive layer or an insulating layer formed on a surface thereof by a thick film paste method.
A method of preparing an aluminum nitride sintered body according to an aspect of the present invention comprises steps of preparing mixed powder containing at least 0.01 percent by weight and not more than 2 percent by weight of carbon powder, at least 0.01 percent by weight and not more than 5 percent by weight of an alkaline earth metal element in terms of an oxide thereof and at least 0.01 percent by weight and not more than 10 percent by weight of a rare earth element in terms of an oxide thereof with a remainder or rest being mainly composed of powder of aluminum nitride, forming a compact with the mixed powder, and forming a sintered body by sintering the compact.
Preferably, the content of carbon in the compact at a temperature of 1500xc2x0 C. is at least 0.01 percent by weight and not more than 0.1 percent by weight in the sintering process.
Preferably, the sintering temperature is not more than 1700xc2x0 C.
Preferably, the mean grain size of the powder of aluminum nitride is at least 0.5 xcexcm and not more than 2.0 xcexcm.
Preferably, the content of oxygen in the powder of aluminum nitride is at least 0.8 percent by weight and not more than 1.5 percent by weight with respect to the weight of the aluminum nitride powder.
A method of preparing an aluminum nitride sintered body according to another aspect of the present invention comprises steps of preparing mixed powder containing at least 0.01 percent by weight and not more than 20 percent by weight of a compound liberating carbon, at least 0.01 percent by weight and not more than 5 percent by weight of an alkaline earth metal element in terms of an oxide thereof and at least 0.01 percent by weight and not more than 10 percent by weight of a rare earth element in terms of an oxide thereof with a rest mainly composed of powder of aluminum nitride, forming a compact with the mixed powder, liberating carbon by heat-treating the compact in a non-oxidizing atmosphere under a condition of at least 150xc2x0 C. and not more than 1500xc2x0 C. in temperature, and forming a sintered body by sintering the heat-treated compact.
Preferably, the compound liberating carbon includes at least one compound selected from a group consisting of polyacrylonitrile, polyvinyl alcohol, polyvinyl butyral, polyethylene terephthalate, glucose, fructose, saccharose, phenol-formaldehyde resin and stearic acid.
Preferably, the content of carbon in the compact at a temperature of 1500xc2x0 C. is at least 0.01 percent by weight and not more than 0.1 percent by weight in the sintering process.
Preferably, the sintering temperature is not more than 1700xc2x0 C.
Preferably, the mean grain size of the powder of aluminum nitride is at least 0.5 xcexcm and not more than 2.0 xcexcm.
Preferably, the content of oxygen in the powder of aluminum nitride is at least 0.8 percent by weight and not more than 1.5 percent by weight with respect to the weight of the aluminum nitride powder.
A method of preparing an aluminum nitride sintered body according to still another aspect of the present invention comprises steps of preparing mixed powder containing at least 0.01 percent by weight and not more than 5 percent by weight of an alkaline earth metal element in terms of an oxide thereof and at least 0.01 percent by weight and not more than 10 percent by weight of a rare earth element in terms of an oxide thereof with a rest mainly composed of powder of aluminum nitride, forming a compact with the mixed powder, and forming a sintered body by sintering the compact in a non-oxidizing atmosphere having a content of at least 10 percent by volume and not more than 100 percent by volume of at least one of carbon monoxide and hydrocarbon.
Preferably, the content of carbon in the compact at a temperature of 1500xc2x0 C. is at least 0.01 percent by weight and not more than 0.1 percent by weight in the sintering process.
Preferably, the sintering temperature is not more than 1700xc2x0 C.
Preferably, the mean grain size of the powder of aluminum nitride is at least 0.5 xcexcm and not more than 2.0 xcexcm.
Preferably, the content of oxygen in the powder of aluminum nitride is at least 0.8 percent by weight and not more than 1.5 percent by weight with respect to the weight of the aluminum nitride powder.
According to the present invention, an aluminum nitride sintered body having stable strength can be obtained by low-temperature sintering employing a sintering aid containing a rare earth element and an alkaline earth metal element, by strictly controlling the amount of the sintering aid and controlling the amount of carbon remaining in the sintered body thereby suppressing grain growth while maintaining excellent basic properties such as high thermal conductivity.
In general, an oxide present in a sintered body reacts with a rare earth element or an alkaline earth metal element blended as a sintering aid to form a rare earth aluminum oxide or an alkaline earth aluminum oxide and form liquid phases on grain boundaries to facilitate sintering. According to the inventors""studies, however, it has been proved that liquid phases are formed in excess if no proper amount of carbon is present to activate mass transfer therethrough, and hence the grain sizes of the sintered body unnecessarily increase as a result.
The present invention has been proposed on the basis of such new recognition that the amount of carbon remaining in the aforementioned aluminum nitride sintered body is closely related to the grain sizes and strength of the sintered body. In other words, low-temperature sintering through liquid phases is enabled while the grain sizes of the sintered body can be suppressed in a desired range by adding carbon to the sintered body to remain therein in a prescribed amount.
According to the present invention, the amount of carbon is controlled to remain in the aluminum nitride sintered body by 0.005 to 0.1 percent by weight, while the contents of the alkaline earth metal element and the rare earth element compound derived from the sintering aid are set to 0.01 to 5 percent by weight and 0.01 to 10 percent by weight in terms of oxides thereof respectively. Thus, the strength of the sintered body can be improved by suppressing grain growth not to increase the grain sizes.
If the amount of carbon remaining in the aluminum nitride sintered body is less than 0.005 percent by weight, oxides cannot be sufficiently reduced due to the insufficient amount of carbon present in sintering. Thus, grain growth of the aluminum nitride sintered body is caused beyond necessity to increase the number of coarse grains, resulting in reduction of the strength of the sintered body. If carbon remains in excess of 0.1 percent by weight, such excess carbon causes deficiency of oxides in the sintered body. Thus, sintering insufficiently progresses under a low temperature of not more than 1700xc2x0 C. In certain examples according to the invention, the remaining carbon content in the sintered body is less than 0.05 percent by weight.
The contents of the alkaline earth metal element and the rare earth element are set in the aforementioned ranges since the density of the sintered body is lowered in low-temperature sintering under a temperature of not more than 1700xc2x0 C. due to deficiency of the sintering aid if the contents of the elements are less than the lower limits of the aforementioned ranges, leading to inferior quality of the sintered body. If the contents of the elements exceed the upper limits of the aforementioned ranges, excess alkaline earth aluminum oxide, rare earth aluminum oxide and alkaline earth rare earth aluminum oxide are deposited on the grain boundaries of the aluminum nitride sintered body, to deteriorate the thermal conductivity.
The alkaline earth metal element preferably includes at least one element selected from a group consisting of Ca, Sr and Ba. The rare earth element preferably includes at least one element selected from a group consisting of Y, La, Ce, Sc, Yb, Nd, Er and Sm. An aluminum nitride sintered body particularly excellent in thermal conductivity and other characteristics can be obtained by employing such alkaline earth metal element and rare earth element.
In the aluminum nitride sintered body, grain growth is suppressed due to reduction of the oxides with carbon as described above, whereby the mean grain size of the sintered body is reduced. In particular, the mean grain size of the sintered body is preferably not more than 3 xcexcm, and more preferably not more than 2 xcexcm. If the mean grain size exceeds 3 xcexcm, the strength and thermal shock resistance of the aluminum nitride sintered body may be so lowered that the aluminum nitride sintered body is unsuitable for application to a radiating substrate for a power module or a jig for semiconductor equipment employed under a particularly strict heat cycle.
The method of preparing an aluminum nitride sintered body according to the present invention is now described. In this method, mixed powder is first prepared by adding an alkaline earth metal element and a rare earth element to aluminum nitride powder as a sintering aid, by at least 0.01 percent by weight and not more than 5 percent by weight and at least 0.01 percent by weight and not more than 10 percent by weight in terms of oxides thereof respectively and further adding carbon or a compound liberating carbon. A compact is prepared from this mixed powder, and this compact is sintered. Thus, an aluminum nitride sintered body containing carbon is obtained.
Alternatively, mixed powder is prepared by adding a sintering aid to aluminum nitride powder in the aforementioned ratios. A compact is prepared from this mixed powder, and this compact is sintered in an atmosphere containing carbon monoxide gas or hydrocarbon gas. Thus, an aluminum nitride sintered body containing carbon is obtained.
The inventive method may be carried out through any of three methods depending on the means of leaving carbon in the obtained aluminum nitride sintered body. In the first method, carbon powder is added in the form of carbon black, coke powder, graphite powder or diamond powder to unsintered material powder of aluminum nitride powder and a sintering aid. The carbon powder must be added by 0.01 to 2 percent by weight. If the amount of the carbon powder is out of this range, it is difficult to control the amount of carbon remaining in the sintered body to 0.005 to 0.1 percent by weight and to improve the strength of the sintered body by suppressing increase of the grain sizes. In certain examples according to the invention, the carbon content in the starting material is more than 0.008 percent by weight and less than 0.05 percent by weight.
In the second method, a compound liberating carbon is employed when sintering aluminum nitride, in place of the aforementioned carbon powder. More specifically, at least one compound is preferably selected from a group consisting of polyacrylonitrile, polyvinyl alcohol, polyvinyl butyral, polyethylene terephthalate, glucose, fructose, saccharose, phenol-formaldehyde resin and stearic acid. With such a compound, which can be dissolved in an organic solvent or water to be thereafter added to/mixed with aluminum nitride powder, carbon can be more homogeneously dispersed in the sintered body as compared with the aforementioned method adding carbon powder. Stearic acid can be added in the form of rare earth salt, as a rare earth element compound forming the sintering aid.
In the second method employing the compound liberating carbon, the compact is heated in a non-oxidizing atmosphere at a temperature of 150 to 1500xc2x0 C., so that carbon is liberated from the compound to contribute to reduction of oxides. The amount of the compound liberating carbon may be in the range of 0.01 to 20 percent by weight, to obtain an effect similar to that in the aforementioned case of directly adding carbon powder.
In the third method, a compact prepared from mixed powder of aluminum nitride powder and a sintering aid is sintered in a non-oxidizing atmosphere containing at least 10 percent by volume of gas selected from carbon monoxide gas and hydrocarbon gas. In this case, oxides in the sintered body can be reduced in a shorter time than those in the first and second methods, due to high reactivity of the gas. According to this method, further, the optimum amount of carbon can be readily left in the sintered body by controlling the composition of the gas in the aforementioned range.
The inventors have observed and studied the sintering process in the inventive method in detail, to find that an aluminum nitride sintered body particularly excellent in strength and the like can be obtained when the amount of carbon contained in the compact or the sintered body at 1500xc2x0 C. in the sintering process is 0.01 to 0.1 percent by weight. If the amount of carbon is less than 0.01 percent by weight in the stage of starting sintering at the temperature of 1500xc2x0 C., the amount of carbon finally remaining in the sintered body is less than 0.005 percent by weight since carbon is further consumed in the later step of reducing oxides. If the amount of carbon is in excess of 0.1 percent by weight in this stage, carbon remains in the grain boundaries of the sintered body to irregularize the color due to heterogeneous transmittance, or sintering incompletely progresses to result in defective sintering density. Therefore, the amount of the remaining carbon at 1500xc2x0 C. must be controlled by setting the speed for increasing the temperature at 1xc2x0 C./min. in the temperature range of 1300 to 1500xc2x0 C. or holding the compact in this temperature range for 1 to 10 hours to sufficiently progress the reaction of Al2O3 +3C +N2xe2x86x922AlN+3CO.
In the aforementioned method according to the present invention, the sintering temperature for aluminum nitride is preferably not more than 1700xc2x0 C. If the sintering temperature exceeds 1700xc2x0 C., grain growth takes place beyond necessity in the aluminum nitride sintered body even if addition of carbon or the like is so controlled that the amount of carbon remaining in the aluminum nitride sintered body is 0.005 to 0.1 percent by weight. Consequently, the mean grain size of the sintered body exceeds 3 xcexcm to lower the strength of the sintered body.
The mean grain size (d50) of the employed aluminum nitride powder is preferably in the range of at least 0.5 xcexcm and not more than 2.0 xcexcm. The term xe2x80x9cmean grain size (d50)xe2x80x9d stands for that grain size having the highest appearance frequency among grain sizes. If the mean grain size exceeds 2.0 xcexcm, it is difficult to obtain a sintered body having fine grains of not more than 3 xcexcm in grain size in particular, due to the excessive initial grain sizes. If the mean grain size of the aluminum nitride powder is less than 0.5 xcexcm, bulk density in powder molding is so increased that it is difficult to increase molding density, and hence the strength of the compact is lowered.
Further, the amount of oxygen contained in the aluminum nitride powder is preferably in the range of at least 0.8 percent by weight and not more than 1.5 percent by weight. If the oxygen content is less than 0.8 percent by weight, the amount of liquid phases formed between oxides and the sintering aid in sintering tends to be insufficient, to lower the sinterability. If the oxygen content exceeds 1.5 percent by weight, the amount of the liquid phases, i.e., grain boundary phases is increased to readily excessively cause grain growth during sintering.
In particular, it has been proved that the adhesion strength of the conductive layer or the insulating layer formed by the thick film paste method is improved in the aluminum nitride sintered body according to the present invention. The first reason for this is that the mean grain size of the sintered body is reduced, particularly to not more than 3 xcexcm, and the second reason is that the wettability of the aluminum nitride grains is improved due to the residual carbon.
The alkaline earth metal element and the rare earth element forming the sintering aid have an effect of improving adhesion between the aluminum nitride grains or adhesion between the aluminum nitride grains and the insulating layer or the conductive layer formed thereon. Compounds of the alkaline earth metal element and the rare earth element are generally present in the vicinity of the grain boundary phases of the aluminum nitride grains in the sintered body. Observing the adhesion strength with respect to the conductive layer or the insulating layer in a microscopic point of view, adhesion between the aluminum nitride grains and the insulating layer or the conductive layer is high in portions where the grain, which are boundary phases of the aluminum nitride grains bonded to each other through the sintering aid, and the conductive layer are in contact with each other. In portions where the aluminum nitride grains are directly in contact with the insulating layer, however, adhesion is conceivably low. Particularly when the mean grain size is greater than 3 xcexcm, coarse grains of aluminum nitride are present in the sintered body to result in sparse distribution of grain boundary phases having high adhesion. Therefore, portions having insufficient adhesion strength tend to arise to readily cause peeling when tensile stress is applied between the aluminum nitride sintered body and the conductive layer or the insulating layer in measurement of peel strength or the like, to result in reduction of the adhesion strength.
According to the present invention, the mean grain size of the aluminum nitride sintered body can be controlled to be small, preferably to not more than 3 xcexcm, as the aforementioned first reason. Thus, compounds of the alkaline earth metal element and the rare earth element are homogeneously distributed on the grain boundaries of such small grains over a wide range with no partial segregation, to further improve the adhesion strength between the aluminum nitride grains and the conductive layer or the insulating layer.
In addition to such distribution of the compounds around the grain boundaries, carbon remaining in the aluminum nitride sintered body reforms the surfaces of the aluminum nitride grains and improves the wettability with respect to the conductive layer or the insulating layer. In particular, the wettability between the metal components and the insulating layer is improved to attain further improvement of the adhesion strength. If the amount of carbon is excessive, however, the sinterability is reduced. Therefore, the amount of carbon remaining in the sintered body is preferably in the range of at least 0.005 percent by weight and not more than 0.1 percent by weight defined as stated above.
Paste employed for the thick film paste method may be any paste generally employed for forming a conductive layer or an insulating layer, such as Ag, Ag paste such as Agxe2x80x94Pt or Agxe2x80x94Pd paste, conductive paste such as Cu paste or Au paste, resistive paste of RuO2, Ru or Bi2Ru2O7, dielectric paste mainly composed of lead borosilicate glass or the like, or high melting point paste of W, Mo, TiN or ZrN.
In order to form the conductive layer or the insulating layer, a thick film layer may be formed by screen-printing the paste on the surface of the aluminum nitride sintered body and heating the same at a prescribed temperature. Alternatively, high melting point paste of W, Nb, TiN or ZrN may be applied to the surface of the unsintered compact to be fired simultaneously with sintering of the compact, to form the conductive layer or the insulating layer.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention.